Arch supporter of comic policemen (8)
I believe the answer is:
keystone
'arch' is the definition.
(thesaurus)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for keystone that I've seen before include "Chief element" , "Central part of a system" , "What tops an arch" , "Piece at the top of a masonry arch" , "Locking piece for arch" .)